1. Field of the Invention
The present invention relates to an improved system of sensing and determining electrical representations of the spatial coordinates of a moveable object, such as a pencil-like stylus ("stylus" and "probe" are used interchangeably herein), and further, to an improved system which accurately determines the position of the object with respect to a two-dimensional coordinate system independent of variations of the object in the third orthogonal dimension, while also measuring the position of the object in this third dimension. The system thus provides greater user comfort and flexibility, and expanded range of features.
2. Description of Prior Art
There are presently available a number of techniques for determining the position of an object in a two-dimensional coordinate system (e.g., an X-Y coordinate system). One such technique employs an electronic stylus operating with a so-called "bit-pad" as an input device, which can be used to provide geometric coordinates to computer graphics and Computer Aidd Design (CAD) systems. Among the most well known of these devices, although no longer in wide use, is the Rand Tablet, which employs a two-dimensional (X-Y) array of conductors carrying a variety of coded pulses that can be detected by a hand-held stylus probe. In this manner, the specific time-dependent pulse pattern induced on the probe at any instant determines the X-Y coordinate position of the probe.
Prior art techniques use electrically-energized conducting surfaces that are directly contacted by or capacitively coupled to a stylus to obtain electrical representations of the stylus position in a manner analogous to a two-dimensional potentiometer. The stylus point corresponds to the wiper contact in an ordinary potentiometer while the conducting surface of finite resistivity corresponds to the resistive element of the potentiometer. In an ordinary potentiometer, the wiper slides back and forth linearly, while in a two-dimensional potentiometric position sensor, the stylus can, of course, move anywhere on the conductive surface. In yet another method of sensing two-dimensional positions, the stylus is attached by rigid or string-like members to a pair of potentiometers, the resistance of which varies with the position of the stylus as it slides along the X and Y axes. In addition, a flexible conductive membrane can be used to contact the primary potentiometric surface when the stylus or other object is pressed against such surface. Examples of some of these techniques are described in L. Reiffel, U.S. Pat. No. 3,617,630; R. D. Bradshaw et al., U.S. Pat. No. 3,423,528; A. B. E. Ellis, U.S. Pat. No. 3,497,617; L. C. Malavard, U.S. Pat. No. 3,632,874; and Japanese Pat. No. 175138.
More recent devices interpret the time of arrival at several microphones of ultrasonic acoustic pulses emitted from a probe to assign digital coordinate codes identifying the probe's position with respect to a two-dimensional or three-dimensional set of axes defined by the microphone array. still another technique utilizes the propogation time of relatively slow moving magnetostrictive pulses launched on an orthogonal array of iron alloy wires. In this technique, the stylus typically contains a coil which detects the passage of the magnetic pulse, thereby permitting its position to be determined. In a variation of this technique, the coil itself launches the flux pulses, which are then detected at the perimeter of the X-Y coordinate plane to determine the position of the stylus.
While many of these previously disclosed techniques of determining X-Y coordinate signals representing the position of a hand-held stylus or probe are useful, each technique has inherent disadvantages peculiar to its principle of operation. Potentiometric methods involving ohmic contact between the stylus and a conducting surface are subject to inaccuracies caused by contamination, physical wear or other damage to the conducting surface or the stylus. Flexible membrane techniques are subject to the tearing or cutting of the membrane. Acoustic techniques are subject to spurious enviromental effects, and the sound source required (usually spark gaps) typically is relatively short-lived or of limited range. Magnetostrictive methods involve special fabrication and calibration techniques, and have a limited acquisition range in the direction orthogonal to the main coordinate plane. Many of the foregoing techniques require opaque structures thereby limiting their usefulness in certain applications. In contrast, as discussed below, the present invention overcomes all of these foregoing disadvantages.